Hi. I am 19 years of age. I have been regular with my menstruation for 2 years until last November when I experienced menstruation twice in just one month.
The first menstruation happened on the first week of November. It was lighter than my normal amount. Then 2 weeks later after the last day of this period, I experienced another menstruation. With slightly heavier than my normal amount. I didn't menstruate in December.

I menstruated first week of January. It lasted until January 7. On January 15, I had protected sex with my boyfriend twice. We were both inexperienced so we both don't know if everything went well. On January 29, we had sex again in the shower, unprotected. It was past 3 weeks since my menstruation so I believe it was safe (based on calendar method) but I was a bit irregular for the previous month so I decided to take Nordette(morning after pill).
A week later, February 6, my menstruation came, or so I thought. But it was so much lighter than my normal, and also compared to my light (first) menstruation last November. I was able to completely soak one pad on the second day, but I didn't even get to soak half of the pad for the next days until it ended on February 11.

I am expecting my menstruation this March 11 or 13, but the other day, March 4, I noticed a single drop of blood on my underwear.

Now, I do not know what is happening. I'm afraid that I'm pregnant, or am experiencing something else in my body. Anyone knows or have experienced similar thing?

Have you taken a pregnancy test? Also, it can be worth it to have a plan B pill on hand if you run out of birth control if you take that. Make sure your partner is using protection if you aren't and you can even both use protection if you want to be super safe. It doesn't sound like you are pregnant though from my experiences I had with my last girlfriend. Get a pregnancy test if you can and go to a doctor if you are very worried and have them help you check.

"A week later, February 6, my menstruation came, or so I thought. But it was so much lighter than my normal, and also compared to my light (first) menstruation last November. I was able to completely soak one pad on the second day, but I didn't even get to soak half of the pad for the next days until it ended on February 11."

This was withdrawal bleeding due to the Nordette hormones leaving your body. It is like a period, but artificially caused, not menstruation.

Nordette can also affect your next few cycles. It works by stopping your natural cycle. After that your body must restart your natural cycle again and there is no predictable way in which it will happen.